Robin Minto

Software development, security and miscellany

HTTP Public Key Pinning (HPKP)

There‚Äôs a new HTTP header on the block - HTTP Public Key Pinning (HPKP). It allows the server to publish a security policy in the same vein as HTTP Strict Transport Security and Content Security Policy.

The RFC was published last month so browser support is limited, supported in Chrome 38, Firefox 35 and newer. However, there are helpful articles from Scott HelmeTim Taubert and Robert Love on the topic and OWASP has some general info on certificate and key pinning in general. Scott has even built support for HPKP reporting into his helpful reporting service - https://report-uri.io/.

Although Chrome and Firefox will honour your public key pins, testing the header is slightly tricky as they haven't implemented reporting yet (as of Chrome 42 and Firefox 38). I spent some time trying to coax both into reporting, working under the assumption that they must have implemented the whole spec right? It seems not.

In writing this, I also wanted to note the command I used to calculate the certifcate digest that's used in the header. In contrast to other examples, this connects to a remote host to get the certificate (including allowing for SNI), outputs to a file and exits openssl when complete.
echo | 
openssl s_client -connect robinminto.com:443 -servername robinminto.com |
openssl x509 -pubkey -noout | openssl pkey -pubin -outform der |
openssl dgst -sha256 -binary | base64 > certdigest.txt

I won't be using HPKP in my day job until reporting support is available and I can validate that the configuration won't break clients. There's great potential here though once the support is available.